Deep Impact’s fireworks continue longer than expected

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A space probe's direct hit on a comet millions of miles from Earth drew oohs and ahs on Monday from astronomers, who said the celestial fireworks show could continue for weeks.

Hours after the Deep Impact mission’s refrigerator-sized Impactor probe collided with Comet Tempel 1, the Flyby mothership still spotted plumes of gas and dust flying out from the impact crater.

"If there are lots of volatiles there, the outgassing will continue to go on for some time ... [perhaps] a matter of weeks," principal scientific investigator Michael A'Hearn said Monday during a post-impact briefing at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

In the weeks and months ahead, scientists will be analyzing the results of the blast to learn more about the composition of comets — which could provide new insights on the formation of the solar system and perhaps even the origin of life on Earth. Findings from the $333 million Deep Impact mission could also help researchers devise strategies for diverting comets that might threaten Earth, said A'Hearn, a professor at the University of Maryland.

The collision, which occurred just before 2 a.m. ET Monday, was the climax of a six-month, 268 million-mile (429 million-kilometer) journey. It was a masterful feat of engineering: Twenty-four hours before the scheduled impact, the 820-pound (373-pound), copper-sheathed Impactor was released by the 1,325-pound (600-kilogram) Flyby craft into the comet's expected path.

Hit or miss?
At first, the autonomous Impactor was on a course that would miss the comet entirely, but it adjusted its trajectory to hit one of the brightest spots on the 3.7-mile-wide (6-kilometer-wide) comet while moving at a relative speed of 23,000 mph (37,000 kilometers per hour).

Deep Impact's mission team compared it to hitting one bullet with another bullet. "It looked a lot like one of our simulation runs," said mission navigator Shyam Bhaskaran.

Impactor’s imaging system sent back pictures of the potato-shaped comet right down to the last three seconds of its existence, while Flyby watched the impact from a front-row seat 5,000 miles (8,000 kilometers) away.

The initial impact came as a bright flash, and within seconds, sprays of ice and dust arced out from the crater. Flyby approached as close as a few hundred miles to analyze the debris, and kept gathering data and images even as it left Comet Tempel 1 behind, 83 million miles (133 million kilometers) from Earth.

The first images had mission team managers jumping and hooting for joy when they flashed on the big screens at the Jet Propulsion Laboratory's mission control. NASA quickly put the imagery on its Web site, attracting a billion hits — more than twice as many as were recorded on the busiest day of the twin-rover mission to Mars, said project manager Rick Grammier.

The science of comets
Comets are "dirty snowballs" that are thought to represent leftovers from the creation of the solar system. Because comets were born in the system’s outer fringes, their cores still possess some of the primordial ingredients, and studying them could yield clues to how the solar system formed 4.5 billion years ago.

Scientists are just beginning to analyze the imagery and spectral data from the Flyby mothership, which should tell them the chemical composition of Comet Tempel 1's interior.

Brown University's Pete Schultz, a member of the Deep Impact science team, said that scientists didn't expect the outgassing to continue as long as it has, based on the simulations conducted before impact. "Now we have to go back to the drawing board and do some more complicated scenarios," he said.

Based on a preliminary analysis, Schultz speculated that the Impactor probe plowed through a soft, dusty surface, then hit a subsurface layer of ice with enough energy to vaporize the ice as well as the probe itself.

No danger to Earth
Collisions with objects such as comets and asteroids are thought to have sparked mass extinctions on Earth in the ancient past, including the
disappearance of the dinosaurs
. Scientists emphasized that Comet Tempel 1 poses no threat to Earth — and that the Impactor blast, though spectacular, had essentially no effect on the comet's course. However, A'Hearn said the mission could represent one small step toward heading off a catastrophic cometary collision if and when the time came.

"The knowledge that comes out of this … is important to understanding how to deflect a comet," he told reporters.

A'Hearn said he was intrigued by the apparent craters that showed up in the last imagery from Impactor. Scientists have seen the surfaces of comet nuclei before — such as Comet Borrelly and
Comet Wild 2
— but "this is the first time we've seen things on the surface that look like impact craters to many of us," A'Hearn said.

Deep Impact's blast was itself expected to create a crater ranging anywhere from the size of a large house to a football stadium and be between two and 14 stories deep. The Planetary Society sponsored a guessing game on the crater’s size — and some members of the science team had even put money down in an office pool. Schultz said he would bet that the crater was bigger than a house, but A'Hearn said it was still too early to determine the size.

"It probably means anybody’s pool can't be paid off for another week," he said.